1. Field of the Invention
The invention relates to a luminance gradation correcting apparatus for correcting a luminance gradation of a video signal.
2. Description of Related Art
In recent years, in association with the increase in size of display apparatuses, there is an increasing need to have a luminance gradation correcting apparatus in order to allow images to be seen more clearly. In a luminance gradation correcting apparatus, usually, a luminance gradation of the video signal is corrected by supplying a video signal to a non-linear amplifier, so as to widen a luminance distribution to a whole extent of the dynamic range.
FIG. 1 shows a conventional luminance gradation correcting apparatus. The luminance gradation correcting apparatus comprises a histogram memory 1, a maximum correction value calculating circuit 2, and a lookup table memory 3. A digital luminance signal is supplied as an input signal to the histogram memory 1 and lookup table memory 3. The histogram memory 1 is a memory for obtaining a luminance distribution of the input luminance signal and has a memory area whose address is designated by each of a plurality of predetermined luminance levels. A frequency is stored in each memory area. That is, each time the luminance signal of one pixel is supplied, the frequency in the memory area of the histogram memory 1 corresponding to the luminance level is increased by 1. All storage contents in the histogram memory 1 are cleared to 0 every predetermined period (one vertical scan period or a period which is an integer multiple of one vertical scan period), so that a new luminance distribution is obtained.
The maximum correction value calculating circuit 2 comprises: a histogram accumulating circuit 2a for sequentially accumulating data in the histogram memory 1 from the data of a low luminance; an accumulation histogram memory 2b for storing a result of the accumulating circuit 2a; and a normalization arithmetic operating circuit 2c for normalizing each data so that the maximum accumulation degree equals the maximum value of an output luminance signal based on the data stored in the accumulation histogram memory 2b. The accumulation histogram memory 2b has a memory area of a frequency which is address designated by each of the luminance levels of the luminance signal in a manner similar to the histogram memory 1.
The lookup table memory 3 stores data obtained by normalizing the storage data in the accumulation histogram memory. An address in the lookup table memory 3 is designated according to the luminance level of the input luminance signal and the luminance level stored in the memory area of the designated address is outputted as a normalized level.
FIGS. 2A to 2C show the luminance converting operation by the conventional luminance gradation correcting apparatus by way of waveforms. One of the addresses in the histogram memory 1 is designated every pixel of the input luminance signal. The value in the memory area of the designated address is increased by 1. It is assumed that the frequency corresponding to the luminance level of the input luminance signal for a predetermined period is detected as shown in FIG. 2A. For the better understanding of the conversion operation, it is assumed that frequencies at luminance levels Y150, Y160, Y170, Y180, Y190, Y200, and Y210 were detected in the histogram memory 1. There are relations of Y150 less than Y160 less than Y170 less than Y180 less than Y190 less than Y200 less than and Y210. Assuming that the frequencies in the predetermined period are equal to 1, 3, 5, 7, 5, 3, and 1 for the luminance levels Y150, Y160, Y170, Y180, Y190, Y200, and Y210, the accumulation frequencies are equal to 1, 4, 9, 16, 21, 24, and 25 for the luminance levels Y150, Y160, Y170, Y180, Y190, Y200, and Y210. That is, as shown in FIG. 2B, as the luminance level increases, the accumulation frequency increases. A normalization coefficient is calculated by the normalization arithmetic operating circuit 2c so that the maximum value of the accumulation frequency equals the maximum value of the output luminance level. The normalization coefficient is multiplied to each data in the histogram memory 1 and a multiplication result is stored into the corresponding memory area in the lookup table memory 3. A relation between the input luminance level and the output luminance level of the lookup table memory 3 is as shown in FIG. 2C. By transmitting the input luminance signal through the lookup table memory 3, the luminance signal is outputted whose gradation has been corrected.
However, as the image carried by a supplied video signal there is a cinesco size image or the like in which an actual image portion is narrow in the vertical direction. In the case of the conventional luminance gradation correcting apparatus, non-image portions (black belts) appear in the upper and lower portions as shown by hatched regions in FIG. 3 in the image mentioned above. If a detecting area of an accumulation histogram is an area surrounded by a dotted line A in FIG. 3 including the non-image portions, therefore, the accumulation histogram has such a characteristic as shown in FIG. 4. That is, the accumulation histogram has a problem that the frequency of the black level in the non-image portion is largely influenced by the frequency of the luminance level in the actual image portion and, if the gradation of the luminance level is corrected based on the accumulation histogram. Thus, there has been a problem of a black floating or the like in which a black level rises in the actual image portion.
It is, therefore, an object of the invention to provide a luminance gradation correcting apparatus which can properly correct a gradation of a luminance level even if a size of actual image portion of an image differs from a size of image shown by a video signal.
According to the invention, there is provided a luminance gradation correcting apparatus comprising: a masking part for allowing only a luminance signal of a pixel in a predetermined detecting range in the vertical direction in an image shown by an input luminance signal to pass; a histogram memory for forming frequency data at each luminance level of the luminance signal outputted from the masking part for every predetermined period and storing it; a correcting part for correcting the luminance level of the input luminance signal based on the frequency data in the histogram memory; and a detecting range setting part for obtaining an accumulation value of the frequency data regarding the luminance signal in which the luminance level lies within a range from a zero level to a non-image display color level in the input luminance signal and setting the predetermined detecting range based on the accumulation value.
According to the luminance gradation correcting apparatus of the invention, since the predetermined detecting range in the vertical direction in the image shown by the input luminance signal is set in accordance with the size of actual image portion of the image, the gradation of the luminance level of the input luminance signal can be properly corrected without being influenced by the luminance level of the non-image portions.